A ballast is an electrical device which is used to provide power to a load, such as an electrical discharge lamp, and to regulate that power, both as to its voltage and current. The ballast provides high voltage to start a lamp, causing an arc to jump from one electrode to the other, by ionizing sufficient plasma (vapor) for the arc to be sustained and to grow. Once the arc is established the ballast allows the lamp to continue to operate by providing proper controlled power flow to the lamp.
In certain types of lamps and other non-resistive loads, the control of power presents special difficulties. For example, one type of high intensity discharge (HID) lamp is a high pressure sodium (HPS) lamp often used for street lighting. In HID lamps an arc is established between electrodes which causes a metallic vapor (xenon, sodium and mercury) to produce radiant energy in the form of visible light, generally without phosphors. The vapor is highly pressurized. HPS lamps are widely used, especially for street lighting, as they are energy efficient (many lumens per watt) and have a long service life, up to 40,000 hours. HPS lamps are also widely used in interior applications where color rendering is not a critical concern.
Ballasts for loads, such as HPS lamps, are generally either magnetic or electronic. Electronic ballasts are presently preferred by many users because, compared to magnetic ballasts, they improve lamp efficiency, reduce light “flicker”, operate more quietly, are better able to regulate the power to the load and are better able to prevent adverse effects on the AC power supply system from the load.
In the operation of HPS lamps, and other HID lamps, the electrodes carry a high-voltage, high-frequency pulse to strike an arc and vaporize the vapor. The ballast must provide sufficient power to the lamp, from the AC source, to provide sufficient open circuit voltage (OVC) to permit polarity reversal without the arc being extinguished (quenched). The AC source is generally a symmetrical, usually sinusoidal, power source at 50/60 Hz.
The superiority of a suitable electronic ballast, compared to a magnetic ballast, is especially evident in the case of HPS lamps, especially those that are aged (have accumulated many service hours). A magnetic ballast is typically an inductance in series with a load, i.e., with an HPS lamp. It includes a “starter”, which is a small pulse generator to strike the arc. The starter is usually connected to a tap on the inductor. The ballast may also include a transformer to match the AC line voltage to the required open circuit voltage (OCV) of the lamp.
The various requirements of a ballast, especially a ballast for HPS lamps, are set forth below, along with comments as to how well magnetic and electronic ballasts presently meet those requirements.    1. The ballast should provide the required lamp illumination level even when the voltage is reduced or raised. The power from the AC source often fluctuates widely. If the lamp were to throw off less light when the power (voltage level) falls, the lamp's illumination might be insufficient. For example a street lamp might not safely light up a street. The fixed impedance of a typical magnetic ballast is in series with the load, i.e., the lamp, and so the ballast, in order to provide a margin of safety against low voltage events, normally provides an excess of power and the lamps normally provide excessive light; This is costly in terms of power consumption and lamp life. When the voltage level is too high (“high line”) even more power is consumed. If the power level declines (“low line”), even momentarily, the open circuit voltage (OCV) may fall below the lamp's requirement, especially for lamps with many hours of service, and the arc may be quenched. After quenching the lamps take a “restrike” interval to cool down before restarting. This may be a problem if a group of lamps has been simultaneously quenched, which may occur when an electrical grid is reconfigured and the voltage momentarily drops to a low-line condition. An ideal ballast would provide a nearly constant effective power equal to the nominal lamp power over the lifetime of the lamp, even under high-line and low-line conditions. Generally, magnetic ballasts fail to provide such power regulation or sufficient energy storage to avoid quenching on very brief voltage dips.    2. Power factor is the ratio of actual power (watts), to volts times amps from the AC power source. Magnetic ballasts typically use a large and expensive capacitor, as a line shunt, to compensate for the ballast's inductance, in order to achieve a higher power factor, for example above 0.95. Magnetic ballasts, when used with HID lamps result in substantial line current distortion, which reduces network efficiency and raises component temperatures, especially in the transformers. Electronic ballasts are able to achieve a high power factor, in the range of 0.95 to 0.99. To do so, the current which is drawn from the AC source should have a sinusoidal wave shape and it should be at most only a few degrees out of phase with the sinusoidal line voltage waveform of the AC supply. For that purpose—a power factor above 0.95—electronic ballasts typically utilize an active power factor correction (APFC) circuit. U.S. Pat. No. 5,515,261 to Bogdan; U.S. Pat. No. 6,169,374 to Chang and U.S. Pat. No. 5,869,937 to Konopka show power supply power factor correction circuits. U.S. Pat. No. 6,169,374 to Chang relates to an electronic ballast for power factor correction at a low cost. It uses a half-bridge inverter to power a fluorescent lamp and mentions both current feedback and voltage feedback. However “feedback” has many meanings and in U.S. Pat. No. 6,169,374 the “feedback” is a direct drive to the load. This is an open loop system. In the present invention, in contrast, a composite current/voltage feedback is fed to an IC (Integrated Circuit) which compares that feedback to a reference. It is a closed loop system.    3. In addition to a power factor correction, to provide a power factor preferably of 0.99 to 1.0, an electronic ballast should also provide the following:            a. The ballast should provide line harmonics which are low, at least less than 20% and preferably less than 5%. The load should appear resistive to the AC line, reducing harmonic current. Total harmonic distortion (THD) is typically calculated using the first 30 harmonics of the fundamental frequency. Some electronic ballasts circuits having active power-factor correction (APFC) also seek to provide low-line current harmonics (see U.S. Patent application 20030001522 to Newman et al. and the references cited therein).        b. The ballast should be highly efficient, preferably above 90% and most preferably above 94% to reduce power consumption.        c. The ballast should provide lamp regulation with sufficient voltage to the lamp during low-line conditions and without a large safety margin, i.e., without excessive voltage during normal operation. Such lamp regulation reduces power consumption compared to less regulated ballasts.        d. The ballast should control power consumption during warm-up of the lamp. The warm-up period is between the start of ignition until the arc has obtained equilibrium. The warm-up time for a HPS lamp is typically several minutes. In the warm-up period the lamp resistance increases from a low value, for example 60 Ohms for a 400 W lamp, to a higher value, for example 400 Ohms, and the required voltage rises for example from 30V (volts) to 90V. The ballast should act as a power limiter during warm up to avoid excessive power system demand.        e. The ballast should provide the required minimum of open circuit voltage (OCV) under all line conditions, especially low-line conditions. The OCV of the lamp depends on its type, wattage, age, etc., but generally, for HPS lamps, it is in the range of 200V. If the OCV is not maintained the lamp arc will likely be quenched.            f. The ballast should provide reliable lamp ignition. In many magnetic ballasts the igniter is a separate external device which is less effective in its speed of restarting the arc (“restrike”) and less robust and less reliable than an igniter which both physically and electrically is integral (internal) to an electronic ballast. Such an integral igniter may provide a rapid restrike, in seconds, if the lamp arc is quenched. The igniter provides the appropriate voltage across the electrodes to initiate spare discharge and sufficient current, at spark discharge, to force a spark to arc transition. For HPS lamps generally the igniter voltage should be over 2000V in the form of voltage pulses, for example pulses of 1 u s (microsecond) duration.            g. The ballast provides a controlled lamp frequency (less than 200 HZ to avoid acoustic resonance) which should be switched so that it is highly symmetric and reverses polarity at least every 10 ms. Generally, in electronic ballasts, the wave form is of a square wave form. The frequency should be below any acoustic resonance (standing pressure waves). If acoustic resonances are generated in an HID lamp, the arc may be distorted, the lamp life shortened and the tube wall may crack.        h. The ballast should provide a lamp crest factor (LCF) close to unity to allow for maximum lamp life, for example 40,000 service hours for an HPS lamp. The lamp crest factor is the ratio of peak current to RMS (average) lamp current at the equilibrium of the lamp. Generally HPS lamp manufacturers accept the highest ratio (peak rms) of 1.4:1 and the lamp crest factor should be below that ratio.        